Field of the Invention
The present invention relates to a laminate sheet structure suitable for use as electrical insulation in such things as motors and transformers. It is especially useful in on-board equipment used in railcars or railways in those jurisdictions that require electrical insulation having a limiting oxygen index (LOI) of greater than 32 percent (%).
Description of the Related Art
New European Standard EN 45545-2, published in 2013, defines fire safety requirements for materials used in railway equipment (rolling stock), including electrical insulation materials for traction transformers and inductors. Three categories of Hazard Level (HL) have been defined. For the highest fire safety level category (HL3), the required LOI for these materials must be higher than 32%.
Thin materials such as aramid electrical insulation papers, that have good mechanical properties (i.e., good tensile strength and elongation-at-break), can have an LOI of less than 32%, and certain inorganic additives to these papers can reduce its mechanical strength. Therefore there is a real need by rail equipment manufacturers for an electrical insulation solution that meets this high LOI limit while maintaining adequate mechanical properties.
To those skilled in the art, the term “mica paper” refers to sheet made with high concentrations of the inorganic mineral mica, generally in an amount that is at least 90 weight percent or greater, with the remainder being a binder that provides some mechanical integrity to paper. However, the resulting mica paper is not a strong paper.
U.S. Pat. No. 6,991,845 to Levit et al. discloses a sheet structure for electrical insulation or flame resistance comprising a barrier ply having a mica-rich face and a mica-poor face and a reinforcing ply containing a saturable backing layer attached to the mica-poor face of the barrier ply. The mica content of the mica-rich face is at least 60 weight percent based on the total components in the mica-rich face. In a preferred embodiment, the mica-rich face has greater than 85 weight percent mica by weight, based on the total components in the mica-rich face.
U.S. Pat. No. 6,991,845 to Levit et al. and U.S. Pat. No. 6,312,561 to Forsten et al. disclose an aramid-mica blended paper made from a homogenous blend of m-aramid fiber, m-aramid fibrids, and mica. Levit et al. further teaches that this “aramid-mica paper” has superior mechanical properties when compared to “mica paper” and can be used without any backing reinforcement.